The present invention is generally directed to modified, hydroxy-substituted or hydroxy-bearing aromatic structures having cytoprotective activity, as well as to a treatment process involving the administration of an effective dosage thereof. More specifically, the present invention is directed to phenolic compounds or catecholic compounds which have been modified by the attachment of a non-fused polycyclic, hydrophobic substituent. Such compounds have been found to possess enhanced cytoprotective activity, as compared to their respective analogs which do not contain such a substituent. This activity may be conferred to a population of cells in a subject upon the administration of an effective dosage of the modified compound.
Cytodegenerative diseases are characterized by the dysfunction and death of cells, this dysfunction or death in the case of neurons leading to the loss of neurologic functions mediated by the brain, spinal cord and the peripheral nervous system. Examples of chronic neurodegenerative diseases include Alzheimer's disease, peripheral neuropathy (secondary to diabetes or chemotherapy treatment), multiple sclerosis, amyotrophic lateral sclerosis, Huntington's disease and Parkinson's disease, Creutzfeldt-Jakob disease and AIDs dementia. Normal brain aging is also associated with loss of normal neuronal function and may entail the depletion of certain neurons. Examples of acute neurodegenerative disease are stroke and multiple infarct dementia. Sudden loss of neurons may also characterize the brains of patients with epilepsy and those that suffer hypoglycemic insults and traumatic injury of the brain, peripheral nerves or spinal cord.
There continues to be a need for treatments that protect cells from cell death resulting from episodes of, for example, disease, trauma, isolation and removal of tissues or cells from the body, or exposure to toxins. This need extends to, among other things: (i) treatments for nerve cell loss associated with chronic or acute neurodegenerative disorders or trauma; (ii) treatments to minimize tissue damage resulting from ischemia where ischemia may occur as a result of stroke, heart disease, a transplantation event, or other event resulting in a cut-off in nutritional supply to tissues; and, (iii) treatments to modulate cell death associated with other degenerative conditions (such as osteoporosis or anemia). The absence of an effective cytoprotective therapy can result in either loss of life or a general decline in the quality of life, including permanent disability with high health care costs to patients, their families and the health care providers.
There have been a number of experimental approaches and targets evaluated to develop drugs for the protection of cells from degeneration. Glutamate, the main excitatory neurotransmitter in the central nervous system, is necessary for many normal neurological functions, including learning and memory. Overactivation of glutamate receptors, however, results in excitotoxic neuronal injury, has been implicated in the pathogenesis of neuronal loss in the central nervous system (CNS) following several acute insults, including hypoxia/ischemia. During brain ischemia caused by stroke or traumatic injury, excessive amounts of the excitatory amino acid glutamate are released from damaged or oxygen deprived neurons. This excess glutamate binds to the N-methyl-D-aspartate (NMDA) receptor which opens the ligand-gated ion channel thereby allowing calcium influx, producing a high level of intracellular calcium which activates biochemical cascades resulting in protein, DNA, and membrane degradation leading to cell death. This phenomenon, known as excitotoxicity, is also thought to be responsible for the neurological damage associated with other disorders ranging from hypoglycemia and cardiac arrest to epilepsy. In addition, there are preliminary reports indicating similar involvement in the chronic neurodegeneration of Huntington's, Parkinson's and Alzheimer's diseases. Accordingly, many pharmaceutical strategies have been assessed which aim to decrease levels of glutamate excess.
Oxidative stress, caused by reactive oxygen species, represents another injury mechanism implicated in many of the same acute and chronic diseases. Reactive oxygen species (e.g., superoxide radical) would cause oxidative damage to cellular components, such as peroxidation of cell membrane lipids, inactivation of transport proteins, and inhibition of energy production by mitochondria.
Glutamate excitotoxicity and oxidative stress may be interlinked; reactive oxygen species formation may occur as a direct consequence of glutamate receptor overstimulation and thus mediate a component of glutamate. Excitotoxicity, in turn, can be reduced by free radical scavengers, including C, Zn-superoxide dismutase, the 21-aminosteroid “lazaroids”, the vitamin E analog, trolox, spin-trapping agents such as phenylbutyl-N-nitrone, and the ubiquinone analog, idebenone which reduce the amount of reactive oxygen species.
Mooradian has reported that certain estrogens have significant anti-oxidant properties in in vitro biochemical assays, but that this effect is not seen with all estrogens. (See, J. Steroid Biochem. Molec. Biol., 45 (1993) 509-511.) Because of the variation in anti-oxidant properties noted by Mooradian in his biochemical assays, he concluded steroid molecules must have certain anti-oxidant determinants which were as yet unknown. Similar observations concerning steroids with phenolic A rings were reported in PCT Patent Application No. WO 95/13076, wherein biochemical assays were used to show anti-oxidant activity. However, the assays used by Mooradian, as well as those used in WO 95/13076, were biochemical assays and, as such, did not directly examine the effects of these molecules on cells. In contrast, Simpkins et al. describe, in U.S. Pat. No. 5,554,601 for example, cell-based assays to determine a method of conferring neuroprotection on a population of cells using estrogen compounds based on demonstrated cell protective effects. As a result, in recent years it has become recognized that estrogen, as well as other polycyclic phenols, may be used for this purpose. (See, e.g., U.S. Pat. Nos. 5,972,923; 5,877,169; 5,859,001; 5,843,934; 5,824,672; and, 5,554,601; all of which are incorporated herein by reference.)
The mechanism by which estrogen compounds bring about a neuroprotective effect is still not fully understood. However, these compounds have been shown to have a number of different physiological and biochemical effects on neurons. For example, estrogen has been shown to stimulate the production of neurotrophic agents that in turn stimulate neuronal growth. Estrogen compounds have also been found to inhibit NMDA-induced cell death in primary neuronal cultures (see, e.g., Behl et al. Biochem. Biophys Res. Commun. (1995) 216:973; Goodman et al. J. Neurochem. (1996) 66:1836), and further to be capable of removing oxygen free radicals and inhibiting lipid peroxidation (see, e.g., Droescher et al. WO 95/13076). For example, Droeschler et al. describe cell free in vitro assay systems using lipid peroxidation as an endpoint in which several estrogens, as well as vitamin E, were shown to have activity. Estradiol has also been reported to reduce lipid peroxidation of membranes (see, e.g., Niki (1987) Chem. Phys. Lipids 44:227; Nakano et al. Biochem. Biophys. Res. Comm. (1987) 142:919; Hall et al. J. Cer. Blood Flow Metab. (1991)11:292). Other compounds, including certain 21-amino steroids and a glucocorticosteroid, have been found to act as anti-oxidants and have been examined for their use in spinal cord injury, as well as head trauma, ischemia and stroke. (See, e.g., Wilson et al. (1995) J. Trauma 39:473; Levitt et al. (1994) J. Cardiovasc. Pharmacol 23:136; Akhter et al. (1994) Stroke 25; 418).
While anti-oxidant behavior is believed to be an important property, a number of other factors are believed to be involved in achieving neuroprotection. As a result, it is to be noted that therapeutic agents selected on the basis of a single biochemical mechanism may have limited generalized utility in treating disease or trauma in patients. For example, in order to achieve an anti-oxidant effect in vitro using estrogen, Droescher et al. used very high doses of estrogens. Such doses, even if effective on neurons in vivo, would have limited utility in treating chronic neurological conditions because of associated problems of toxicity that result from the prolonged use of these high dosages.
In addition to the issues related to compound toxicity, consideration must also be given to the ability of a particular compound to reach the target site, which in some applications is controlled by the ability of the compound to cross the blood-brain barrier. The blood-brain barrier is a complex of morphological and enzymatic components that retards the passage of both large and small charged molecules, and thus limits the access of such molecules to cells of the brain. Furthermore, not only must the compound be capable of reaching the target site, but it must also do so in a state or configuration which enables it to carry-out its designated function.
In view of the foregoing, it can be seen that a need continues to exist for the identification of compounds which have demonstrated biological efficacy in protecting humans from the consequences of abnormal cell death in body tissue; compounds which are capable of crossing the blood-brain barrier and which are suitable for administration in dosages which are non-toxic. This identification requires continuing advances in the understanding of the structural requirements for compositions capable of inducing neuroprotection, which in turn provide the basis for designing novel drugs that have enhanced cytoprotective properties while at the same time have reduced adverse side effects.